Methods and apparatus for determining soil matric potential are known in the art. Soil matric potential (soil water potential) is soil moisture tension, an indicator of the ability of water to move within soil.
Tensiometers have been used in the past for soil moisture measurement. Tensiometers measure how tightly water is being held by soil. See, for example, U.S. Pat. No. 3,898,872 to Skaling et al. (incorporated herein by reference). The tensiometer disclosed in the Skaling et al. patent includes a liquid filled vessel having walls with a porous section to be placed in the soil. Pressure in the tube is measured to determine the moisture content of the soil. Another tensiometer is described in commonly assigned U.S. Pat. No. 5,644,947 to Hubbell et al. (incorporated herein by reference).
A conduction type soil matric potential sensor is disclosed in U.S. Pat. No. 4,137,931 to Hasenbeck (incorporated herein by reference). The Hasenbeck patent discloses a matric potential sensor including a housing having a porous wall adapted to be located underground, and spaced electrodes carried in the housing and coupled to a source of electrical current. Current flow varies as a function of moisture content of soil. Moisture content varies as a function of the matric potential of the soil surrounding the housing in response to moisture transmission through the porous housing into the surrounding soil.
Apparatus for measuring matric potential are typically buried in the ground. Sediment samples are routinely collected, for characterization activities, at construction sites, dam sites, landfills, hazardous waste disposal sites, and other sites. The samples are collected by drilling or by other means. These sediment samples are often tested for moisture content, but typically not for matric potential. When such samples are tested for matric potential, they may be tested for matric potential in the dry range using a thermocouple psychrometer. Thermocouple psychrometers are used in the laboratory to calculate matric potential by using the Peltier Effect of adiabatic cooling to determine humidity, and then relating the determined humidity to calculate matric potential. Thermocouple psychrometers require considerable skills to maintain calibration, and therefore generally yield poor results.